The aim of this project is to investigate the mechanisms by which genomic DNA-damage by mutagenic/carcinogenic chemicals inactivates cells (a combination of cell death and reproductive sterilization) and impairs population growth. In this project, study of the mechanisms of cytotoxic effects from damage to DNA are examined because much information is available about the chemical modification of this molecule and the effects of specific lesions on DNA replication and base mispairing (mutation). We will examine three monofunctional chemicals (MNNG, BPDE, and 4-NQO) that produce distinctly different and well-defined damage to DNA, damage that is repaired by different cellular processes and leads to cytotoxic responses of different magnitude. The effects of chemical damage on the dynamics of population growth will be studied in cultured fibroblastic cells from two species, man (NSFL 7010) and mouse (C3H 10T1/2), that appear to represent opposite ends of the spectrum of sensitivity to cytotoxicity from chemically-induced DNA damage. Three general categories of cellular response to DNA injury will be assessed (death, reproductive failure, and slowed or transiently blocked cycle transit) in response to varying chemical doses administered to cells at specific points in synchronous cycle transit. We hypothesize that two major consequences of DNA damage result in qualitively and quantitatively different cytotoxic responses by replication-dependent machanisms: a. prevention of complete replication of the genome is associated with high yields of dead and reproductively sterilized cells when damage is inflicted at any time during S phase and b. inaccurate replication of actively expressed housekeeping genes impairs cell viability by mutation of specific genes or by imparing gene expression and is maximally effective when damage occurs in early S phase. Dynamic changes in cell populations will be assessed with new methods employing flow cytometry and cell sorting, combined with classic techniques.